1. Field of the Invention
The present invention relates to high-brightness liquid crystal display devices that are high in optical use efficiency and excellent in viewing-angle characteristics.
2. Description of the Related Art
Liquid crystal display devices are widely used as those devices for use in visually displaying a variety of kinds of images including still images and video images.
Such liquid crystal display devices of this type are basically arranged so that two (a pair of) substrates at least one of which is made of transparent glass or else are spatially stacked over each other with a liquid crystal layer sandwiched between them to thereby constitute a so-called liquid crystal panel, which devices are classified into two forms, one of which is for performing turn-on/off operations of a specified picture element or "pixel" by selective application of a voltage to various electrodes as formed on the substrate of the above-noted liquid crystal panel to constitute the pixel, and the other of which is for performing turn-on/off operations of certain pixel by forming an active element for pixel selection along with the various electrodes and then selecting this active element.
Especially the liquid crystal display devices of the latter type are called active-matrix devices, which offer several technical advantages including increased contrast performance and high-speed displayabilities or the like and, for the very reason, are inevitable as "de facto standard" display modules among presently available liquid crystal display devices.
Prior known active-matrix liquid crystal display devices include those of what is called the "vertical electric field" scheme for applying between an electrode formed on one substrate and an electrode formed on the other substrate an electric field for use in changing the optical orientation or alignment direction of the liquid crystal layer, and others of the so-called "lateral electric field" scheme (also known as in-plane switching type or "IPS" scheme) for letting the direction of an electric field being applied to the liquid crystal layer extend in substantially parallel to substrate surfaces.
The various types of liquid crystal display devices are each designed to include a light source device (generally called a "back-light" unit) for illumination of the liquid crystal panel from the back side thereof. Currently available back-light unit typically include those of the side-edge scheme with more than one lamp (linear light source such as a fluorescent tube or light-emitting diode) disposed on the lateral side of a light guide plate used, and others of the direct downward scheme with a lamp disposed immediately beneath the liquid crystal panel.
Especially for notebook personal computers requiring reduced thickness and light weight, it is a general approach to employ back-light units of the side edge scheme.
FIG. 14 is a diagram depicting an exploded perspective view of one exemplary structure of a liquid crystal display device. In FIG. 14, "SHD" designates a shield casing (also known as metal frame) made of a metal plate; WD denotes a display window; INS1-3 indicate dielectric sheets; PCB1-3 represent printed circuit boards (PCB1 is a drain-side circuit board for use as an image signal driver circuit board, PCB2 is a gate-side circuit board, and PCB3 is an interface circuit board); JN1-3 are joiners for electrical connection between associated ones of the circuit boards PCB1-3; TCP1, TCP2 are tape carrier packages; PNL, a liquid crystal panel; GC, rubber cushion; ILS, optical shielding spacer; PRS, prism sheet; SPS, optical diffuser sheet; GLB, light guide plate; RFS, reflection sheet; MCA, lower-side casing (mold frame) as formed by all-at-a-time machining; MO, opening of lower-side casing MCA; ;LP, fluorescent tube; LPC, lamp cable; GB, rubber bush for support of the fluorescent tube LP; BAT, both-side adhesive tape; BL, back-light unit consisting essentially of a linear light source (fluorescent tube) LP and light guide plate GLB and the like, wherein diffuser plate members are stacked or laminated over one another to assemble the liquid crystal display module MDL.
Although in the illustrative liquid crystal display device the reflective sheet RFS is disposed on the lower surface (back plane) of the back-light BL with the diffuser sheet SPS and prism sheet PRS being laminated on the upper surface thereof, another diffuser sheet SPS may be further stacked on the prism sheet PRS where appropriate. In addition, while in this arrangement a single unit of linear light source LP is laid out along one side of the light guide plate GLB with wedge-like cross-section, there are also known a large-screen size device which is designed to employ two or more light sources together or alternatively the one that makes use of a flat plate-shaped light guide plate having its two parallel opposite sides along each of which one or a plurality of linear light sources are installed. Additionally the remaining components are explained by adding to the drawing their parts names along with the reference characters thereof
The liquid crystal display module MDL is structured from two types of accommodation/support members such as the lower side casing MCA and shield casing SHD and is arranged by integrally engaging the shield case SHD and the lower side case MCA together, wherein the former is made of metal to receive therein the dielectric sheets INS1-3 and circuit boards PCB1-3 plus liquid crystal panel PNL whereas the latter receives therein the back-light BL that consists essentially of the fluorescent tube LP and light guide plate GLB plus prism sheet PRS and others.
The image signal line driving circuit board PCB1 has a built-in integrated circuit (IC) chip for use in driving each pixel of the liquid crystal panel PNL; the interface circuit board PCB3 has components mounted thereon, including an IC chip for receiving an image signal(s) from an external host and also a control signal or signals such as a timing signal or the like along with a timing converter TCON for processing the timing to generate a clock signal.
The clock signal as generated by the timing converter is supplied to the IC chip mounted on the image signal line drive circuit board PCB1 via a clock signal line CLL railed on the interface circuit board PCB3 and image signal line driving circuit board PCB1.
The interface circuit board PCB3 and image signal line driving circuit board PCB1 are multilayer lead pattern substrates, wherein the clock signal line CLL is formed as an internal lead of the interface circuit board PCB3 and image signal line driving circuit board PCB1.
Note that the liquid crystal panel PNL is arranged so that the drainside circuit board PCB1 for driving TFTs and gate-side circuit board PCB2 plus interface circuit board PCB3 are connected by the tape carrier packages TCP1, TCP2 while associative ones of such circuit boards are connected together by the joiners JN1, 2, 3.
FIG. 15 is a diagram pictorially representing a sectional view of a multilayer structure of the liquid crystal display device shown in FIG. 14, wherein the liquid crystal layer LC is sandwiched between the lower substrate SUBI and upper substrate SUB2 to constitute the liquid crystal panel, the lower substrate SUB1 and upper substrate SUB2 having their outer surfaces on which a double-layer structure consisting of a lower phase difference plate PHD1 and lower polarization plate POL1 and a bilayer structure of an upper phase difference plate PHD2 and upper polarization plate POL2 are stacked respectively.
This liquid crystal panel has its back surface (rear plane) on which the back-light BL is placed while letting an optical sheet consisting of a lamination of the diffuser sheet SPS and prism sheet PRS be inserted between the liquid crystal panel and the back-light BL.
The back-light BL used is of the side-edge scheme type, wherein the linear light source (fluorescent tube or light-emitting diode) LS and reflector sheet LP are disposed along one lateral side of a light guide plate GLB-W having a wedge-like cross-section. Although in this back-light BL two fluorescent tubes LP are used to achieve increased brightness, these may be replaced with a single one or three or more ones on a case-by-case basis. Where necessary, a structure may be employed which includes one or two or more fluorescent tubes as disposed along each side of the opposite sides of a flat planar light guide plate. Additionally the back-light BL has its back face on which the reflector film RFS is disposed.
FIG. 16 is a pictorial representation for explanation of one example of a groove layout of the prism sheet in FIG. 15, wherein the lower side prism sheet PRS1 elongates in the lateral direction (rightward/leftward direction=horizontal direction: X) for collecting those rays of light coming from the back-light BL together in the lengthwise direction (upward/downward direction=vertical direction: Y) whereas the upper side prism sheet PRS2 extends in the up/down direction to collect together light rays from the back-light BL in the lateral direction thereby letting resultant light fall onto the back surface of the liquid crystal panel PNL at sharp or acute angles thereto.
FIG. 17 is a diagram for explanation of the significance of a front-face brightness relative to the upper and lower viewing angles of a display screen due to the presence or absence of the prism sheet, which indicates front-face brightness values of respective cases including a case of providing a single prism sheet, a case of providing two ones, and a case of using only diffuser plate (in the absence of any prism sheet).
As shown in the same drawing, provision of a single prism sheet PRS results in an increase in brightness when compared to the case of avoiding the use of such prism sheet PRS (with the diffuser plate only). And, it would readily occur to those skilled in the art that providing two prism sheets PRS results in a further increase in brightness on the front side.
Note here that the back-light of this type has been disclosed in "SID 96 DIGEST" at pp. 753-756.
Liquid crystal display devices are increasing in size every year, which in turn requires that back-light modules associated therewith likewise increase in dimension. In addition to such size increase, a need to achieve higher brightness or luminance is getting more strict, which necessitates the development of advanced back-light units with enhanced light intensity (increased brightness) through improvements in use efficiency of rays of light concerned.
A method has been employed for using the above-noted prism sheet or sheets to increase the brightness or luminance intensity by collection of those rays of incident light diagonally travelling with respect to the liquid crystal panel toward a front face side. Another method is also known which makes use of a recently developed optical film (such as for example "D-BEF" film.TM. as manufactured by SUMITOMO-3M Company) as designed to let output light of a light source have polarization characteristics for permitting penetration of only specific rays of the polarized light for re-use purposes while causing the remaining rays to be reflected off.
However, the related art approach of employing more than one prism sheet is such that when an operator or user moves to diagonally look at the liquid crystal panel at certain offset angles to the normal to the front surface thereof, there will possibly be observed the phenomenon that the brightness behaves to decrease and thereafter increase again.
FIG. 18 is a pictorial representation for explanation of the brightness characteristics in a liquid crystal display device using a prism sheet. Light L that has reached the prism sheet PRS is such that when passing through the prism plane, most rays L.sub.D of it is changed in optical travel path toward the direction of the liquid crystal panel thereby improving the liquid crystal panel's front face brightness. However, the remaining light components L.sub.L that have been mirror-reflected at the prism plane or have left the prism sheet to outgo at large angles relative to the liquid crystal panel can result in occurrence of either drop-down or re-increase of the brightness of the liquid crystal panel when viewing diagonally the liquid crystal panel.
FIG. 19 is a brightness characteristic diagram for explanation of one example of a relation of white-display brightness versus upper/lower viewing angles of a liquid crystal panel using a prism sheet. As indicated by curve "a" in this drawing, as the upper/lower viewing angles of the liquid crystal panel vary from zero degrees (front view field) to a diagonal direction, the resulting brightness (cd/m.sup.2) behaves to gradually decrease to finally reach its minimal value at or near .+-.30 degrees and then acts to again increase thereafter. This can disadvantageously serve to lower the. on-screen image recognizability or viewability over the entire area of such liquid crystal panel.
The above-discussed optical film aimed at improvement of brightness by utilizing polarized light rays is faced with a problem that its brightness enhance ratio is different depending upon different viewing angles, which would result in a decrease in on-screen image viewability in a manner similar to that stated above.